I am pretty sure the the CAAD9 is stiffer.
Cannondale is releasing 2008 models over the next month- you may be able to get discounts on 2007 bikes, but I have heard the 2008 bikes will be cheaper (and lower specc'ed). It is the same frame, check the cdale forum for more info.

I think CAAD9 Optimo would go for around $1600, but I bet someone else will post the cheaper price really fast. I have a CAAD9 with Dura-Ace components, and my previous ride was a Specialized S-Works E5 frame (I think it's Columbus SLX tubing...). I think CAAD9 is noticeably stiffer: you'll feel the road buzz, but I like it because the power transfer just feels so direct. Weight-wise, it's pretty comparable, but CAAD will feel very fast. Personally, CAADs are aluminum frames on rabies, go for the Cannondale

go for the caad9

I have ridden the E5, Caad4 Caad5, Caad7, 05 Tarmac, several Allezs, and my brother's Caad 9. All are good frames, but the Caad 9 wins hands down. Obviously go with whichever fits you better, but the Caad9 is truly the best-bang-for the buck of any current race frame IMO. Wicked stiff, surprisingly comfortable, and stays true to it's crit roots while being a better all-arounder than the caad 4/5 geometry. Admitted bias: I really like my older Klein and the Caad 9 is the closest modern frame out there.

Sorry can't help you with pricing - I had no reason to pay attention and there are too many variables. But, I'd wait for clearance prices seeing as the '08 stuff will be rolling in soon.